Typically, when the temperature of a process fluid flowing in process tubing needs to be automatically measured, a thermocouple is used. To improve the accuracy of the measurement, the thermocouple is placed as close as possible to the fluid. However, the thermocouple is placed external to the tubing because there are electrical leads attached to the probe that need to be easily accessible in order to read the temperature measurement. Since the thermocouple is not physically in contact with the fluid, the reaction time is too slow and the measured temperature is not accurate.
One method used in the past to increase the reaction time is to machine a thin-walled depression into the wall of a fitting, such as a T-shaped fitting. Potting compound is then used to firmly hold a thermocouple in the depression. In this configuration, the thermocouple still does not make contact with the fluid, however the reaction time is decreased since the thermocouple is surrounded on three sides by the fluid, and the potting compound is chosen to be thermally conductive. Unfortunately, the depression in the fitting is expensive to machine. Also, it is also difficult to remove and replace the thermocouple should the thermocouple fail.
Using a bayonet-type spring-loaded thermocouple facilitates removal and replacement of the sensor. But this type of thermocouple is difficult to place close to the process fluid and consequently, has a slow reaction time when sensing changes in the temperature of the flow. This device also requires manufacturing an expensive thermal-weld in which to install the thermocouple.
VCR fittings are commonly used in fluid and gas flow systems. A typical assembly will usually comprise two glands engaging and sealing on either side of a gasket, held together by connecting male and female nuts; or a gland engaging and sealing on one side of a gasket and a connector body engaging and sealing on the other side of the gasket, secured by a nut. One approach to the current problem designed for use with VCR components is to thread a thermocouple through the gasket and weld the gasket to the sheath of the thermocouple. However, the drawback to this design is that the thermocouple is not reusable, and thus it cannot be cleaned.
What is needed is a thermocouple probe design that can react quickly and accurately to temperature changes and is easily removable and replaceable.